Chapter 14
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Chapter 14. The Arthropods: Blueprint for Success. Evolutionary Perspective. Metamerism modified by tagmatization Chitinous exoskeleton Paired, jointed appendages Ecdysis Ventral nervous system Coelom reduced to cavity around gonads Open circulatory system Complete digestive tract

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Chapter 14

Chapter 14

The Arthropods: Blueprint for Success


Evolutionary perspective

Evolutionary Perspective

  • Metamerism modified by tagmatization

  • Chitinous exoskeleton

  • Paired, jointed appendages

  • Ecdysis

  • Ventral nervous system

  • Coelom reduced to cavity around gonads

  • Open circulatory system

  • Complete digestive tract

  • Metamorphosis often present


Classification and relationships to other animals

Classification and Relationships to other Animals

  • Ecdysozoans

    • Cuticle, ecdysis, loss of epidermal cilia (figure 14.2)

  • Monophyletic with five subphyla (table 14.1)

    • Chelicerata, Crustacea, Hexapoda, Myriapoda, Trilobitomorpha (entirely extinct)


Chapter 14

Figure 14.2 Evolutionary relationships of the arthropods to other animals.


Chapter 14

Table 14.1


Metamerism and tagmatization

Metamerism and Tagmatization

  • Metamerism evident externally

    • Segmental body wall

    • Segmental appendages

  • Metamerism reduced internally

    • No septa

    • Most organs are not metameric

  • Tagmatization obvious

    • Specializations for feeding, sensory perception, locomotion, and visceral functions


Learning outcomes section 14 3

Learning Outcomes: Section 14.3

  • Describe the structure of the arthropod exoskeleton or cuticle.

  • Assess the influence the exoskeleton has had on the evolution of the arthropods.


The exoskeleton

The Exoskeleton

  • Exoskeleton or cuticle

    • External jointed skeleton

  • Functions

    • Structural support

    • Protection

    • Prevents water loss

    • Levers for muscle attachment and movement

  • Covers all body surfaces and invaginations

  • Secreted by epidermis (hypodermis)


The exoskeleton1

The Exoskeleton

  • Epicuticle (figure 14.3)

    • Lipoprotein

    • Impermeable to water

    • Barrier to microorganisms and pesticides

  • Procuticle

    • Chitin

      • polysaccharide

    • Outer procuticle hardened by sclerotization or deposition of calcium carbonate

    • Inner procuticle less hardened and flexible

      • Articular membranes at joints (figure 14.4)

  • Modifications include sensory receptors

    • Sensilla


Chapter 14

Figure 14.3 Arthropod exoskeleton.


Chapter 14

Figure 14.4 Modifications of the exoskeleton.


The exoskeleton2

The Exoskeleton

  • Growth accompanied by ecdysis (figure 14.5)

    • Enzymes from hypodermal glands begin digesting old procuticle (a, b).

    • New procuticle and epicuticle secreted (c, d).

    • Old exoskeleton splits (e)

    • Calcium carbonate deposition and/or sclerotization hardens new exoskeleton (f).


Chapter 14

Figure 14.5 Events of ecdysis.


The hemocoel

The Hemocoel

  • Embryonic blastocoel

  • Internal cavity for open circulatory system

    • Fluids bathe internal organs.

    • Exchange of nutrients, wastes, and sometimes gases

  • Not a hydrostatic compartment


Metamorphosis

Metamorphosis

  • Radical change in body form and physiology as an immature (larva) becomes an adult.

    • Reduces competition between adult and immature stages


Subphylum trilobitomorpha

Subphylum Trilobitomorpha

  • Dominant life form from Cambrian period (600 mya) to Carboniferous period (345 mya)

  • Substrate feeders

  • Three tagmata: head, thorax, and pygidium

  • Three longitudinal sections

  • Biramous appendages


Chapter 14

Figure 14.6 Subphylum Trilobitomorpha (Saukia sp).


Subphylum chelicerata

Subphylum Chelicerata

  • Spiders, mites, ticks, horseshoe crabs

  • Two tagmata

    • Prosoma

      • Eyes

      • Chelicerae

        • Often chelate

        • Usually feeding appendages

      • Pedipalps

        • Sensory, feeding, locomotion, reproduction

      • Walking legs

    • Opisthosoma

      • Digestive, reproductive, excretory, and respiratory organs


Class meristomata

Class Meristomata

Figure 14.7 A eurypterid, Euripterus remipes.

  • Subclasses

    • Eurypterida

      • Extinct giant water scorpions

        (figure 14.7)


Class meristomata1

Class Meristomata

Figure 14.8a Limulus polyphemus.

  • Subclass Xiphosura

    • Horseshoe crabs

      • Limulus (Atlantic Ocean and Gulf of Mexico)

      • Book gills

        • Gas exchange between blood and water

      • Reproduction

        • Dioecious

        • External fertilization


Chapter 14

Figure 14.8b Ventral view of Limulus.


Class arachnida

Class Arachnida

  • Spiders, mites, ticks, scorpions

  • Arose from ancient euryptrids

  • Very early terrestrial groups

    • 280-400 mya

    • Exoskeleton was preadaptation for water conservation.


Form and function

Form and Function

  • Carnivores

    • Chelicerae to hold prey or as fangs

    • Gut

      • Foregut

        • Cuticular

        • Pumping stomach

      • Hindgut

        • Cuticular

        • Water reabsorption

      • Midgut

        • Noncuticular

        • Secretion and absorption


Form and function1

Form and Function

  • Excretion

    • Coxal glands

      • Paired sacs bathed in blood of body sinuses

      • Homologous to nephridia

      • Excretory pores at base of posterior appendages

    • Malpighian tubules

      • Blind ending diverticula of gut tract

      • Empty via digestive tract

    • Uric acid


Form and function2

Form and Function

  • Gas Exchange

    • Book lungs

      • Paired ventral invaginations of body wall

      • Gas exchange between air and blood across book lung lamellae

    • Tracheae

      • Branched, chitin-lined tubes

      • Open at spiracles along abdomen


Chapter 14

Figure 14.9 An arachnid book lung.


Form and function3

Form and Function

  • Circulation

    • Open with dorsal contractile vessel

    • Pumps blood into tissue spaces of hemocoel

    • Returns to dorsal vessel via ostia

  • Nervous system

    • Ventral with fusion of ganglia


Form and function4

Form and Function

  • Senses

    • Mechanoreceptors

      • Modifications of exoskeleton

      • Sensilla respond to displacement.

    • Chemical sense

      • Pores in exoskeleton

    • Vision

      • Eyes detect movement and changes in light intensity.

Figure 14.10 An arthropod seta (a) and an eye (ocellus) (b).


Form and function5

Form and Function

  • Reproduction

    • Dioecious

    • Indirect sperm transfer

      • Male deposits spermatophores, which are transferred to the female.

    • Courtship rituals common

    • Copulation occurs in spiders via modified pedipalp of male.

  • Development

    • Direct


Order scorpionida

Order Scorpionida

  • Prosoma

    • Shieldlike carapace

  • Opisthosoma

    • Preabdomen

    • Postabdomen (“tail” with sting)

  • Courtship prior to mating

  • Oviparous, ovoviviparous, or viviparous


Chapter 14

Figure 14.11 (a) Hardrurus arizonensis (b) External anatomy.

(a)

(b)


Order araneae

Order Araneae

  • Spiders

  • Prosoma

    • Chelicerae with poison glands and fangs

    • Pedipalps leglike

      • Sperm transfer in males

    • 6-8 eyes

  • Opisthosoma

    • Connected to prosoma via pedicel

    • Swollen or elongate

    • Visceral functions and spinnerets


Chapter 14

Figure 14.12 External structure of Argiope.


Chapter 14

Figure 14.13 Prosoma of a spiderling.


Order araneae1

Order Araneae

  • Silk

    • Protein

    • Repeating sequence of glycine and alanine

    • Beta sheet

    • Stored as gel prior to spinning

    • Chemical modification when forced through spinnerets

  • Webs, line retreats, safety lines, wrapping eggs, dispersal of young (ballooning)


Chapter 14

Figure 14.14 Members of the family Araneidae are the orb weavers.


Order araneae2

Order Araneae

  • Feeding

    • Insects and other arthropods

    • Hunt or capture in webs

    • Paralyze prey

      • May wrap in silk

    • Inject enzymes into prey body wall

  • Two spiders are venomous to humans.


Chapter 14

Figure 14.15 (a) Black widow spiders (Lactrodectus mactans) has a neurotoxic venom. (b) Brown recluse spiders (Loxosceles reclusa) have a histolytic venom.

(b)

(a)


Order araneae3

Order Araneae

  • Reproduction

    • Complex behaviors

      • Chemical, tactile, and visual signals

    • Male’s pedipalps enlarged into embolus

      • Male deposits sperm on web and collects with pedipalps.

      • Transfers sperm to female during mating

    • Female deposits eggs in silk case.

      • In webbing, a retreat, or carries with her


Order opiliones

Order Opiliones

Figure 14.16 Order Opiliones (Leiobunum sp).

  • Harvestmen or daddy longlegs

  • Prosoma broadly joins opisthosoma

  • Legs long and slender

  • Omnivores

  • External and internal digestion


Order acarina

Order Acarina

  • Mites

    • Prosoma and opisthosoma fused and covered by single carapace

    • 1mm or less

    • Free-living

      • Herbivores or scavengers

        • Many pest species

    • Ectoparasites

      • Chigger (Trombicula)

      • Follicle mite (Demodex)

Figure 14.17 Dermatophagoides farinae is common in homes and grain storage areas.


Order acarina1

Order Acarina

  • Ticks

    • Ectoparasites in all life stages

    • Up to 3cm

    • Females lay eggs after engorging with blood.

    • Important in disease transmission

      • Rocky Mountain spotted fever

      • Lyme disease


Chapter 14

Figure 11.18 Ixodes scapularis transmits the bacteria that causes Lyme disease.


Class pycnogonida subphylum cheliceriformes

Class Pycnogonida (Subphylum Cheliceriformes?)

  • Sea spiders

  • Marine

  • Feed on cnidarian polyps

  • Dioecious

  • Molecular, developmental, and morphological characters are being used to reevaluate taxonomic status.

Figure 14.19 Class Pycnogonida


Subphylum crustacea

Subphylum Crustacea

  • Crayfish, shrimp, lobsters, crabs, copepods cladocerans and others

  • Almost all are aquatic

    • Terrestrial isopods and crabs are exceptions.

  • Two pairs of antennae

  • Biramous appendages (figure 14.20)


Chapter 14

Figure 14.20 Crustacean body form. (a) External anatomy. (b) Biramous appendages.


Class malacostraca

Class Malacostraca

  • Crabs, lobsters, crayfish, shrimp, krill, amphipods, isopods

  • Order Decapoda

    • Largest order

    • Shrimp, crayfish, lobsters, crabs


Class malacostraca1

Class Malacostraca

  • Crayfish external structure

    • Cephalothorax

      • Fusion of head and thorax

      • Covered dorsally and laterally by carapace

      • Sensory, feeding, locomotion

    • Abdomen

      • Muscular “tail” in crayfish

      • Locomotor and visceral functions in others

    • Paired appendages

      • Serially homologous (derived from a common ancestral pattern)


Chapter 14

Figure 14.22 External structure of a male crayfish.


Chapter 14

Figure 14.23 Serial homology of crayfish appendages.


Class malacostraca2

Class Malacostraca

  • Crayfish internal structure

    • Digestive system

      • Complete with foregut, midgut, and hindgut

    • Respiratory system

      • Gills attach at base of cephalothoracic appendages.

      • Lie within gill chamber between carapace and lateral body wall

      • Second maxilla circulates water.

    • Circulation

      • Open

      • Dorsal heart and major arteries

      • Blood enters hemocoel, and gills before returning to pericardial sinus around heart.


Chapter 14

Figure 14.24 Internal structure of a crayfish.


Class malacostraca3

Class Malacostraca

  • Ventral nervous system

    • Cephalization and centralization

    • Supraesophageal and subesophageal ganglia process sensory information and control head appendages.

    • Segmental ganglia

  • Sensory structures

    • Antennae

    • Compound eyes

    • Statocysts

    • Chemoreceptors

    • Proprioceptors

    • Tactile setae


Class malacostraca4

Class Malacostraca

  • Endocrine system

    • Ecdysis, sex determination, color change

      • X-organs

        • Neurosecretory tissues in eyestalks

        • Molt-inhibiting hormone

          • Target Y-organ

      • Y-organs

        • Base of maxillae

        • Releases ecdysone when molt inhibiting hormone is not present and ecdysis occurs

    • Androgenic glands (males)

      • Promotes development of testes and male characteristics


Class malacostraca5

Class Malacostraca

  • Excretion

    • Antennal (green) glands in crayfish

    • Maxillary glands in others

    • Homologous to coxal glands of arachnids

  • Reproduction

    • Dioecious

    • Mating after female molts

      • Fertilized eggs attach to female’s pleopods

      • Others have planktonic larvae


Chapter 14

Figure 14.25

(a) Nauplius larva of a barnacle. (b) Zoea larvae of a crab.

(a)

(b)


Order isopoda

Order Isopoda

  • “Pillbugs”

  • Aquatic and terrestrial

  • Dorsoventrally flattened

Figure 14.26a Order Isopoda.


Order amphipoda

Order Amphipoda

  • Laterally compressed

  • Crawl or swim on sides

  • Beach-hoppers modified for jumping

Figure 14.26b Order Amphipoda.


Class branchiopoda

Class Branchiopoda

  • Fairy shrimp

    • Temporary ponds

  • Brine shrimp

    • Great Salt Lake

  • Cladocera

    • Freshwater water fleas

    • Large carapace

    • Parthenogenesis common

  • Flattened, leaflike appendages

Figure 14.27 Order Cladocera.


Class maxillopoda

Class Maxillopoda

Figure 14.1 Subclass Copepoda.

  • Subclass Copepoda

    • Most abundant crustaceans

    • Important in marine and freshwater food webs

    • First antennae modified for swimming


Class maxillopoda1

Class Maxillopoda

  • Subclass Thecostracea, Infraclass Cirripedia

    • Barnacles

    • Marine

    • Monoecious

      • Nauplius and cypris larvae

      • Cypris larva settles and metamorphoses into sessile adult.

    • Some parasites


Chapter 14

Figure 14.28 Class Maxillopoda, Infraclass Cirripedia.

(a) Internal structure of an acorn barnacle. (b) A stalked barnacle (Lepas).


Further phylogenetic considerations

Further Phylogenetic Considerations

  • Diverse body forms and lifestyles of Arthropoda arose from single ancestor.

  • Crustaceans very successful in aquatic habitats

  • Chelicerata

    • First terrestrial arthropods

    • Account for evolution of many water conserving features of the phylum

      • Exoskeletal, excretory, and respiratory adaptations


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